1. Field of the Invention:
The embodiments of this invention relate to a method of driving a semiconductor laser device and an apparatus for driving a semiconductor laser device.
2. Description of the Prior Art:
When driving a semiconductor laser device to obtain a laser beam, a forward current I.sub.F is supplied to a pn junction in the laser device. The relationship between the forward current I.sub.F and the optical output P.sub.O of a semiconductor laser device is not linear. As the forward current I.sub.F supplied to a semiconductor laser device increases, the laser device begins to oscillate a laser beam at a certain level of the current (a threshold current I.sub.th). As the forward current I.sub.F increases further, the laser output P.sub.O also increases. FIG. 2 shows an example of the relationship between the forward current I.sub.F and the optical output P.sub.O of a semiconductor laser device.
The level of a threshold current I.sub.th and the rate of change in an optical output P.sub.O with respect to the changes in a forward current I.sub.F are not constant. The level varies according to an ambient temperature or to the individual semiconductor laser device. Such a rate of change in an optical output P.sub.O is called a differential efficiency .eta..
When a semiconductor laser diode is driven at an optical output of a constant level, the system shown in FIG. 3 is generally used. In this system, a semiconductor laser diode 1 is driven by a current source 4 which is controlled by the output of an amplifier 3. The optical output of the laser diode 1 is monitored by a photodiode 2, and the optical output is converted into a voltage by a resistor 5. The voltage is applied to an input terminal of the amplifier 3. The system shown in FIG. 3 constitutes a negative feedback loop. A reference voltage V.sub.ref is applied to the other input terminal of the amplifier 3 so that the optical output of the laser diode 1 is controlled to have a predetermined level corresponding to the reference voltage V.sub.ref.
The system shown in FIG. 3 is used for obtaining a constant optical output of a predetermined level. In certain applications of a semiconductor laser diode, such as laser printing or optical communication, a semiconductor laser diode produces a constant optical output of a predetermined level and is turned on and off at a high speed. FIG. 4 shows an example of a system used for such an application.
The system of FIG. 4 comprises, in addition to the components in the system of FIG. 3, an analog switch 7 and a capacitor 8 which constitute a sample-hold circuit. The system of FIG. 4 further comprises a high-speed switch 6 and a buffer amplifier 9. The operation of the system of FIG. 4 will be described as follows. First, the switch 6 is set to the right position, and the switch 7 is turned on to obtain an optical output of a predetermined level corresponding to the reference voltage V.sub.ref. Thereafter, the switch 7 is turned off. At this stage, the voltage applied to the amplifier 9 is held at the voltage level of the capacitor 8 so that the driving current supplied to the laser diode 1 is constantly maintained. Then, the switch 6 is turned off and on at a high speed and a high-speed switching of the optical output at a constant level results.
With the system shown in FIG. 4, it is difficult to maintain the optical output at a constant level for a long period of time since the voltage of the capacitor 8 is an analog value. The system illustrated in FIG. 5 has been proposed to solve this problem, i.e., the system can maintain the optical output at a constant level for a long period of time.
The system of FIG. 5 is not provided with the sample-hold circuit (the analog switch 7 and capacitor 8) and the buffer amplifier 9 which are used in the system of FIG. 4. The system of FIG. 5 comprises an up/down counter 11, a D/A converter 10, and an oscillator 12. In the system of FIG. 5, the amplifier 3 functions as a comparator. When the output of the comparator 3 is "HIGH", the up/down counter 11 counts up the output pulses of the oscillator 12. In contrast, the output of the comparator 3 is "LOW", in contrast, the counter 11 counts down the output pulses of the oscillator 12. The output of the counter 11 is converted into an analog value corresponding to the forward current I.sub.F by the D/A converter 10. When the switch 6 is set to the right position, the forward current I.sub.F from the current source 4 is supplied to the semiconductor laser diode 1 to drive the laser diode 1. The system of FIG. 5 constitutes a negative feedback loop so that the optical output of the laser diode 1 is regulated at a constant value corresponding to the reference voltage V.sub.ref. An error which corresponds to one pulse of the output of the oscillator 12 may occur in the level of the optical output.
Since digital codes are used in the feedback loop, the system of FIG. 5 can maintain the light output at a constant level for a long period of time. In the system of FIG. 5, however, the D/A converter 10 should have high resolution in order to maintain an optical output which is regulated to a constant level with high accuracy because the I.sub.F -P.sub.O characteristics of a semiconductor laser diode are not linear.
The I.sub.F -P.sub.O characteristics of a semiconductor laser diode are such that, if a forward current I.sub.F is less than the threshold current I.sub.th, laser oscillation does not occur. Accordingly, if the forward current I.sub.F is less than the threshold current I.sub.th, the system cannot be used for control in driving the laser diode. For this reason, the quantization error of the optical output P.sub.O is greater than the quantization error of the forward current I.sub.F, and a smaller accuracy for controlling the optical output P.sub.O result.
The above-mentioned problem will be discussed in more detail. FIG. 12(a) shows a relation between an input (digital codes) and an output (forward current I.sub.F) of a D/A converter which are usually used in a conventional system such as shown in FIG. 5. FIG. 12(b) shows a relation between optical outputs P.sub.O of a laser diode and digital codes in the conventional system of FIG. 5 which employs a D/A converter having the linear characteristics as shown in FIG. 12(a). The digital codes corresponding to values lower than the threshold current I.sub.th of a laser diode do not contribute to the optical output of the laser diode. Therefore, shown in FIG. 12(b), the range of the codes which are effective in practical use is restricted, and a reduced effective resolution of the D/A converter 10 results. In FIG. 12(b), "W.sub.q " indicates a quantization width of the optical output P.sub.O.
In the system of FIG. 5, an optical output of a desired level can be obtained by applying a reference voltage V.sub.ref which corresponds to the desired optical output level and closes the negative feedback loop (hereinafter, this process is referred as "calibration"). As the calibration is not directly related to the driving operation of the laser diode 1, it is difficult to employ the system of FIG. 5 in an apparatus in which the optical output level should be changed very frequently. For example, in an optical magnetic disk recording apparatus, the optical output is changed into three levels (reading, erasing, and recording levels).